Object becomes transparent when motion blur is enabled (Cycles)
Motion blurred objects are inherently semi-transparent if they're moving fast enough, because they do not cover any area of the frame for the duration of the exposure. Motion blur results because an object moves during the exposure and thus "partially" covers an area larger than itself. For large or slow moving objects, there will be a large region that they obscure for the entire exposure, thus the transparent blur only appears on the edges. However, if an object is sufficiently small and moving fast enough, it can cover the distance of its own width during a single exposure. At this point, there is no region that is covered for the entire exposure and the object appears to become transparent.

This is not a limitation of CG, it happens with real objects too. See, for example, how the car in this photograph is almost invisible except for the lights which are able to overwhelm objects behind them: